Geometry Corner Mesh
Your mesh has some n-gons and the Subdivision Modifier doesn't like it. :)
Make your mesh only from quads and it will subdivide properly.

Other little issues:


Messed up scale - Solve through Object > Apply > Scale (and check normals afterwards)
Non-manifold faces - some faces are going through another. This has to be repaired manually. Either merge the vertices, or move the faces farther away.


Quads vs. N-gons with Subdivision Modifier



Apply your object's scale to avoid problems in the future.



Here is your .blend back, repaired so you can compare :).